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Porver9two4
Joined: 22 Jan 2004 Posts: 104 Location: United Kingdom
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Posted: Wed Oct 10, 2018 1:05 am Post subject: Weak spark problem - likely cause? |
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Can anyone offer some wisdom on where to look for the likely cause of the latest ailment to maroon my 924......
I've finally got the fuel system properly sorted (having been into every single component of the fuel system) only to find myself stranded again with a 924 that wouldn't start...... This time it's ignition
I had a 'no start' episode the other day, investigation of which showed no spark. Changed the king lead, still no spark. Turned out the rotor arm in the distributor had burned out, with obvious arcing and burning from the central contact out. A replacement from my spares stash saw fat spark restored and I was on my way again - all fixed. Visual inspection showed no issues with the cap or anything else. Happy 924 days once more....
EXCEPT, yesterday I again had a 'no start' episode Luckily I still had a spark tester on board and this revealed a very weak spark - a bump start was enough for it to burst into life and once running it got me home. I parked it at home then tried to start it again - no dice, spark too weak. Came to it this morning.... big fat spark again and it burst into life immediately.
So clearly something is failing or breaking down as it is getting hot. What is the most likely cause of this? Anyone experienced these symptoms before? Is this a distributor cap issue? or a coil issue? |
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924RACR
Joined: 29 Jul 2001 Posts: 8806 Location: Royal Oak, MI, USA
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Posted: Wed Oct 10, 2018 3:11 am Post subject: |
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Well... if it will bump-start easily, seems as if the starter is drawing down the voltage enough that the ignition box won't work, right? Had a bit of that with my Turbo this summer. Can be impacted by bad grounds, of course...
Of course, when hot it takes more juice to crank, as well... _________________ Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype |
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Porver9two4
Joined: 22 Jan 2004 Posts: 104 Location: United Kingdom
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Posted: Wed Dec 05, 2018 9:40 pm Post subject: |
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I'm still really struggling with this.....
I haven't found the root cause. I'm still getting an issue with the spark weakening as the car gets hot. I can drive it for an hour flat out and it runs and drives fine, start getting into heavy traffic though and it'll start to flutter a little as the spark weakens. If I then switch off, it won't restart as the spark is too weak, although leave it to cool right off (like overnight) and a great big fat spark is back.....
I have found I can improve the spark enough to get it to start by hot wiring direct off the battery +ve to the coil +ve and juicing the coil with 12v, which gives a decent spark, but I can't really use it like it is.....
So far I've replaced the plugs, leads, rotor arm and cap and checked voltage at the coil (which all measures correctly) and have taken to carrying a spare coil, in case it's that but the last time it wouldn't start and I jury rigged the spare coil in it didn't make a huge amount of difference.
It's the most frustrating sort of fault as obviously something is failing, but not yet failed and that makes it so hard to trace. Start it from cold and it's perfect.
Help! I am not sure where to hunt...... |
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KDJones2000
Joined: 14 Sep 2010 Posts: 322 Location: Phoenix, AZ
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Posted: Wed Dec 05, 2018 11:11 pm Post subject: |
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Well it could be in your ignition switch.
When you are cranking it is supposed to deliver 12V directly from the battery to the coil. When not, 12V goes through another path to the coil.
Trace your circuits and see if there is an issue. _________________ 1987 924S SPEC car
Team Toofah Racing
1988 944 Turbo S |
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924RACR
Joined: 29 Jul 2001 Posts: 8806 Location: Royal Oak, MI, USA
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Posted: Thu Dec 06, 2018 12:36 am Post subject: |
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I eventually broke down and swapped out my ignition box... not sure if your car uses the same part, mine (82 931 US-spec) is part # 191 905 351 A Ignition Switch Unit.
BUT
It MUST be installed on the factory heat sink, using heat conductive paste. As you would use to install a CPU on a motherboard. The original paste on my original ignition box had dried out and was no longer conducting properly.
I am actually still using an older, used ignition unit, though I got a new one from a VW supplier (it's a common part)... but I will note that the new replacement came with a smaller heat sink riveted on, which I don't know would be sufficient for the 924.
If you do indeed have this style of ignition unit (on the front left fender), you might try cleaning and reinstalling it to the heat sink with new silver conductive paste from a computer store, see if that helps...
Good luck! _________________ Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype |
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staticsan
Joined: 19 Jan 2009 Posts: 450 Location: Sydney, Australia
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Posted: Thu Dec 06, 2018 6:06 pm Post subject: |
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Could you post a pic of your ignition unit? I've had issues with mine, but I am puzzled by your description of a distinct heatsink (yes, I know what heatsink compound is).
Wade. _________________ '82 British NA - Which I think I've been very lucky with! |
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924RACR
Joined: 29 Jul 2001 Posts: 8806 Location: Royal Oak, MI, USA
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staticsan
Joined: 19 Jan 2009 Posts: 450 Location: Sydney, Australia
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Posted: Fri Dec 07, 2018 2:13 pm Post subject: |
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Oh! That's really different from mine - and I think I know why. Your's is a Turbo. _________________ '82 British NA - Which I think I've been very lucky with! |
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safe
Joined: 18 Mar 2017 Posts: 587 Location: Sweden
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Posted: Sat Dec 08, 2018 3:36 am Post subject: |
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That's an early one up to 79 I think. It sits on the fire wall close to the distributor.
Later, and turbos have a bigger unit located behind the left headlight on the wheel well.
Right? |
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924RACR
Joined: 29 Jul 2001 Posts: 8806 Location: Royal Oak, MI, USA
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Posted: Sat Dec 08, 2018 7:59 am Post subject: |
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safe wrote: |
That's an early one up to 79 I think. It sits on the fire wall close to the distributor.
Later, and turbos have a bigger unit located behind the left headlight on the wheel well.
Right? |
Actually what you describe is the opposite of what I've seen; the one pictured above (which I had heat sink issues with) is on my '82 931. _________________ Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype |
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lwebster
Joined: 21 Oct 2018 Posts: 11 Location: Ann Arbor, MI
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Posted: Thu Jan 03, 2019 1:06 pm Post subject: |
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Interestingly, my '81 turbo came with a spare module. Must be worthwhile to have a spare in glovebox |
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Ozzie
Joined: 12 Mar 2005 Posts: 4448 Location: Townsville, Qld. Australia
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Posted: Fri Jan 04, 2019 12:03 pm Post subject: |
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My '84 na had a bad/no spark issue and found there was no earth on the ecu and it earthed through the mounting screws which were corroded.
I fitted a dedicated earth to the body and no further problems.
There is no earth wire on the plug to my ecu. _________________ Porsche 924 1984 (UK import) NA
Its AUTO and its BLACK
Montego Black on black/red
Engineer of Electro/Mechanical Systems Maintenance |
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